Single-photon detection is a useful technique in the fields of spectroscopy, biological imaging, astrophysics, and quantum-information processing. Single-photon detection may also be useful for communication systems in which received signal levels are very low. The technique can allow detection of trace elements or molecules, biological processes or specimen organization, distant stars, quantum computational results, and weak signals that would not be detected with conventional semiconductor photodetectors.
Although different types of single-photon detectors have been developed, most have limitations when applied to imaging applications. For example, photomultiplier tubes and avalanche photodiodes have been used very successfully as individual detectors for single-photon detection in the visible region of the optical spectrum. However, their response to photons in the infrared region (wavelengths longer than about 900 nanometers) deteriorates appreciably compared to the visible region. Additionally, photomultiplier tubes are expensive and not suitable for incorporating into compact imaging arrays having sub-millimeter pixel sizes. Although avalanche photodiodes are more suited for incorporating into compact imaging arrays, their quantum efficiency is limited to about 85% over a narrow portion of the visible spectrum and drops to about 50% at 400 nanometers (400 nm) and 900 nm. It is also difficult to incorporate avalanche photodiodes, configured for single-photon detection, in imaging arrays and achieve low dark-count rates (less than 500 counts per second).
Other recently-developed single-photon detectors include the transition edge sensor (TES) and microwave kinetic inductance detector (MKID), however these devices have temporal resolutions over several nanoseconds (ns) and microseconds (μs), respectively. Recently, superconducting-nanowire single-photon detectors (SNSPDs) have been operated in a linear array, but it was found that the array size would be limited to tens of detector elements. Their pixel-number limitation and long delay lines between the SNSPDs make these devices unsuitable for large-area two-dimensional arrays of pixels having a high fill factor.